1. Field of the Invention
This invention relates to an optical path length compensating optical system in an optical apparatus, and in particular to an optical path length compensating optical system suitable as the alignment optical system of an exposure apparatus used in the photolithography process of the manufacture of semiconductors.
2. Related Background Art
Along with the tendency of semiconductor integrated circuits toward higher integration, the exposure apparatuses used in the photolithography process have been experiencing the remarkable shift from the contact system or the mirror projection system to the reduction projection exposure system which enables more minute transfer to be accomplished. This system is a system in which the pattern of a reticle is reduction-transferred to a wafer by a reduction projection type exposure apparatus (hereinafter referred to as the "stepper"). Usually, a reticle pattern of 100 mm.times.100 mm to 75 mm.times.75 mm is reduced to 1/10-1/5 by a projection lens and is exposed on a wafer having a diameter of 100 mm to 150 mm in exposure areas of 10 mm.times.10 mm to 15 mm.times.15 mm in succession while involving the moving operation of a stage fed at the interferometer standard called the step and repeat, whereby transfer is accomplished.
In the semiconductor manufacturing process, ten and several times of exposure and process treatment are repeated even for the same exposure area and in that case, there is required the so-called alignment in which a new reticle pattern is optically superposedly transferred onto the pattern transferred during the preceding process, at desired accuracy. In this case, ideally, it is desirably to superpose an alignment mark included in the new reticle pattern upon an alignment mark included in the pattern on the wafer transferred during the preceding process, with substantially the same wavelength as that during the exposure, through a reduction projection lens, and shift to the next exposure. A method in which alignment is thus effected for each exposure in order to ensure the accuracy and only the portion exposed once is aligned is called the die-by-die alignment, and in this die-by-die alignment, it is necessary to bring the alignment optical system into coincidence with the alignment mark on the reticle. This alignment mark is provided around the pattern on the reticle, but the size of the pattern differs depending on the design of the semiconductor circuit and therefore, to bring the alignment optical system into coincidence with that alignment mark, it is requisite to move the alignment optical system in accordance with the different size of the pattern.
Further, to realize highly accurate die-by-die alignment, it is necessary not only to confirm the coincidence between the alignment marks of the reticle and the wafer, but also to photoelectrically detect the alignment at high accuracy to thereby accomplish the alignment automatically. For this purpose, it is necessary to detect the alignment signal at the pupil position of the alignment optical system as a Fourier conversion surface, and it is desirable that the pupil position be maintained invariable in spite of the movement of the alignment optical system.
On the other hand. the technique of so-called optical trombone in which a pair of plane mirrors obliquely disposed in a parallel light beam are parallel-moved to correct the length of the optical path has heretofore been used in a projection inspection apparatus or the like and is well known, and a projection type exposure apparatus in which the conjugate relation between the image and the pupil is maintained by the use of this optical trombone optical system is already known, for example, from Japanese Laid-Open Patent Application No. 150924/1983 (corresponding to U.S. patent application Ser. No. 469,015 filed on Feb. 23, 1983now U.S. Pat. Ser. No. 4,592,625). The alignment optical system described in this prior art is designed such that only the variation in the length of the optical path for the change in the position of the alignment mark in one direction (x direction) on the reticle can be corrected, and this leads to the following disadvantages.
In the die-by-die alignment, confirmation of alignment is generally effected by a light of substantially the same wavelength as the exposure wavelength and therefore, furing the alignment, the alignment mark on the then used wafer is exposed, and disappears after the process and thus, is not reusable. Accordingly, during the next alignment, it is necessary to effect alignment by the use of an unexposed mark in y direction adjacent to that alignment mark which has disappeared. Therefore, when the pattern size and the position of the alignment mark during each exposure process are taken into consideration, it is desirable that the objective in the alignment optical system which is opposed to the reticle be designed so as not only to be movable with respect to an axis (x direction) but also to be panarily (two-dimentionally) with respect to two axis (x and y directions). Moreover, in order that highly accurate alignment may be accomplished in that case, it is necessary that the position of the pupil of the alignment optical system as a Fourier conversion surface be stably maintained. However, the aforedescribed well-known alignment optical system is designed such that the trombone optical system effectively acts on only the movement of the objective in one direction (x direction) and therefore, during the alignment in a direction perpendicular thereto (y direction), the entire alignment optical system must be moved in y direction, and this leads not only to the bulkiness of the apparatus but also to a disadvantage that quick and precise alignment operation becomes impossible. Also, where each one set of alignment optical systems is disposed on the left and right of the reticle to effect alignment not only in x and y directions but also in the direction of rotation, the respective alignment optical systems can be independently moved in x direction to thereby accomplish alignment freely, but in y direction perpendicular thereto, the left and right alignment optical systems must be moved as a unit, and this leads to a disadvantage that the design of the semiconductor circuit regarding the alignment mark is greatly limited thereby.